Field of the Invention
The present invention relates to an atomizer and a combustion device provided with the atomizer. Specifically, the present invention relates to the twin-fluid atomizer for atomizing the liquid fuel (spray fluid) with the aid of gas (spray medium for atomization), and the combustion device provided with the atomizer as described above.
Description of the Related Art
The combustion device such as the combustor for gas turbine is demanded to achieve high environmental performance for coping with diverse types of fuel, in other words, fuel diversification, and for reducing unburnt carbon in the exhaust gas and nitrogen oxide (NOx).
Coping with the fuel diversification, the gas turbine of some type employs the dual fuel combustor for selectively using the fuel gas such as natural gas, and the liquid fuel such as light oil and heavy oil A in accordance with the fuel supply situation. The dual fuel combustor is configured to be operable in the case of stagnation of the fuel supply of any one of the gas fuel and the liquid fuel by using the other one.
Aiming at the high environmental performance, the combustor for gas turbine is demanded to reduce emission of the nitrogen oxide (NOx), carbon monoxide (CO), and particulate matters mainly from combustion. As for reduction in the nitrogen oxide (NOx), it is essential to lessen thermal NOx generated by oxidization of nitrogen in the air at high temperatures. The lean fuel premixed combustion method for combusting the fuel which has been preliminarily mixed with excessive air is effective for lessening the thermal NOx. The lean fuel premixed combustion method is designed to supply air more than necessary for complete combustion of the fuel for mixing the fuel and air before combustion. Resultant uniform low-temperature combustion may suppress generation of the thermal NOx.
It is preferable to apply the lean fuel premixed combustion method to the combustor for gas turbine in the use of the liquid fuel for reducing generation of NOx. For the purpose of performing the lean fuel premixed combustion, it is necessary to mix the air with the liquid fuel having the liquid fuel constituent preliminarily vaporized before combustion. If the liquid fuel is atomized in the aforementioned state, the surface area of the liquid fuel per weight is increased to facilitate vaporization. Upon application of the method to the combustion device of another type, atomization of the liquid fuel will increase its surface area per weight to accelerate the combustion reaction, thus reducing CO and unburnt carbon as a result of embers of combustion. Therefore, expedience of atomization of the liquid fuel becomes one of essentials for reducing NOx, CO, and unburnt carbon.
One of the atomizers for atomizing the liquid fuel may be of twin-fluid type, which is configured to supply both the liquid fuel as the spray fluid and the spray medium for atomization such as air and vapor, which are mixed for atomization. Generally, compared with the pressure atomization method for atomizing only the spray fluid, the twin-fluid atomizer exhibits higher atomization performance in the case of spray with large capacity. In spite of change in the supply amount of the liquid fuel, fluctuation in the atomization performance may be lessened by adjusting the supply amount of the spray medium. As a result, the aforementioned type has been generally employed for the combustion device such as the gas turbine combustor with variable combustion load.
The twin-fluid atomizer (hereinafter referred to simply as an “atomizer”) is demanded to enhance the atomization performance, and also reduce consumption of energy required for spray by lessening the supply pressure of the spray fluid, and consumption and supply pressure of the spray medium. The atomizer configured to lessen the energy consumption for spray, and to allow highly efficient atomization has been under examination.
Japanese Unexamined Patent Application Publication No. Sho62-186112 exemplifies the atomizer of internal mixing type, configured to mix the spray fluid with the spray medium in the space (mixing chamber) within the flow passage of the atomizer so that the resultant mixed fluid is jetted through a large number of outlet holes. The atomizer is configured to mix the spray fluid and the spray medium in the mixing chamber for atomization. By jetting the mixed fluid at high speeds through the outlet holes, velocity difference between the mixed fluid and the ambient gas will cause the shearing force to act on the spray fluid in the mixed fluid for atomization.
Japanese Unexamined Patent Application Publication No. Hei9-239299 exemplifies the atomizer configured to allow mixtures of the spray fluid and the spray medium to flow from opposite directions so that those mixtures collide with each other around the outlet holes for expediting atomization. The disclosed atomizer is configured to jet the mixed fluid through the outlet holes into a fan-like spray. Because of the fan-like spray shape, the atomizer is called fan-spray type. Compared with the atomizer disclosed in Japanese Unexamined Patent Application Publication No. Sho62-186112, the atomizer of the above-described type is configured to cause collision of the mixed fluids around the outlet holes to accelerate mixture between the spray fluid and the spray medium. The jet of the mixed fluids through the outlet holes is formed into the fan-like shape, resulting in increased boundary surfaces with the ambient gas.
Japanese Unexamined Patent Application Publication No. Hei4-254109 exemplifies the atomizer configured to have a hole through which the spray fluid is jetted to the internal center, and a two-stage orifice formed at the downstream side of the hole. The spray medium is supplied from the upstream side of the two-stage orifice so as to mix the spray fluid and the spray medium by means of the orifice. The atomizer of this type allows high-speed supply of the spray fluid to the orifice so that the spray medium is drawn with the aid of high momentum of the spray fluid. The suction effect derived from the spray medium is capable of lessening the supply pressure of the spray medium. The spray medium is mixed with the spray fluid from the outer periphery so as to pass through the orifice. This makes it possible to narrow the jetting angle of the mixed fluid jetted through the outlet holes.
The atomizer disclosed in Japanese Unexamined Patent Application Publication No. Sho62-186112 is configured to mix the spray fluid and the spray medium for atomization in the mixing chamber so that the mixed fluid is jetted through a plurality of outlet holes. The fluid is atomized mainly through mixture in the mixing chamber and the shearing force generated by difference in the flow velocity between the mixed fluid jetted through the outlet holes at high speeds and the ambient gas. If the flow velocity of the mixed fluid is decelerated in the mixing chamber, the spray fluids which have been atomized by the surface tension thereof will be combined to generate coarse particles. In particular, upon collision of the spray fluid against the wall surface of the mixing chamber, the liquid film is generated on the wall surface, and separated from the spray medium.
In the case of high velocity of the spray fluid around the wall surface, the shearing force will separate the liquid film from the wall surface, which will be mixed with the spray medium again. On the contrary, in the case of low flow velocity of the spray fluid around the wall surface, the liquid film will grow. If the aforementioned liquid film reaches the outlet hole, and is jetted outside the atomizer, it is formed into droplets at the high flow velocity through the outlet hole. Mixture with the spray medium is stagnated, and the droplet has a large particle size. Compared with the mixed fluid flowing at the center of the outlet hole, the spray fluid in the mixed fluid jetted from circumference of the outlet hole may have a larger particle size.
The atomizer disclosed in Japanese Unexamined Patent Application Publication Nos. Sho62-186112 and Hei9-239299 is configured to mix the spray fluid and the spray medium within the atomizer. It is therefore necessary to keep each supply pressure of the spray fluid and the spray medium in the fixed range. The spray fluid in liquid form generally has high density, and exhibits small volume fluctuation upon increase in the supply pressure. Meanwhile, the spray medium in gaseous form generally has low density, and exhibits large volume fluctuation upon increase in the supply pressure. As a result, consumption of energy for increasing the supply pressure of the spray medium is higher than that of the spray fluid. In general, increase in the supply pressure both of the spray fluid and the spray medium results in high atomization performance. However, as the energy consumption is reduced, the supply pressure of the spray medium will be restrictive.
The twin-fluid atomizer disclosed in Japanese Unexamined Patent Application Publication No. Hei4-254109 is configured to mix the spray medium and the spray fluid from the periphery at the upstream side of the orifice. The spray fluid is unlikely to adhere to the partition of the atomizer. It is possible to suppress generation of the coarse particle owing to the liquid film formed on the wall surface. However, the orifice has to be formed in a jetting hole of the spray fluid on the axis of the hole. Because of restriction of the area with which the orifice is formed, the above-structured atomization is not suitable for the high capacity usage.
An object of the present invention is to provide the atomizer that is improved in atomization performance by preventing the liquid film formed around the wall surface of the mixing chamber from reaching the outlet hole of the atomizer, and the combustion device with the atomizer.
Another object of the present invention is to provide the atomizer that is suitable for mixing the spray fluid and the spray medium by facilitating dispersion of the spray fluid in the mixing chamber, and the combustion device with the atomizer.
The aforementioned atomizer is of type with less restrictive arrangement between the supply hole for supplying the spray fluid to the mixing chamber and the outlet hole for jetting the mixed fluid outside the mixing chamber, which is suitable for the high capacity usage.